Efficient operation of modern refineries requires that refiners introduce larger and larger portions of heavier, poorer quality crude oils into refinery feedstocks. The widespread use of poorer quality crude oils in turn requires efficient processes and catalysts for converting heavy feedstock bottoms such as atmospheric and vacuum residua (hereafter "resid") to more valuable, lighter molecular weight products.
Resid conversion is difficult because resid feedstocks typically contain high concentrations of metals such as nickel, iron and vanadium as well as high concentrations of hetero atoms such as nitrogen and sulfur. Many of these metals and hetero atoms deactivate or poison hydrotreating catalysts used in resid conversion units. Poisoning of the catalyst often leads to the need for frequent catalyst additions or changeouts which impact unit availability and throughput.
Resid conversion also is difficult because resid feedstocks contain a large asphaltenic fraction that produces insoluble carbonaceous material when the feedstock is heated. These insoluble materials are known as Shell Hot Filtration Solids (hereafter "SHFS" or "Shell Solids") and are formed in greater degree at higher operating temperatures. Shell Solids impact operability by fouling and plugging system components. SHFS-induced operability problems generally occur as SHFS levels exceed about one weight percent within a resid conversion reactor. Formation of these solids, therefore, often results in feedstock or temperature operating limitations.
The operational impact of catalyst poisoning and SHFS formation has caused refiners to implement a variety of processes and catalysts in an effort to more efficiently convert resid feedstocks to more valuable, lighter molecular weight products.
In some cases, refiners have converted resid by employing multistage processes using different catalysts in each stage. For example, U.S. Pat. No. 4,297,242 to Hensley discloses a two-stage process in which the first stage employs a large pore, high surface area demetallation catalyst having a Group VIB and/or Group VIII metal deposed thereon and in which the second stage employs a relatively smaller pore second stage catalyst having a Group VI metal deposited on a catalytically-active alumina support. In Hensley's system, demetallation primarily occurs in a first stage reactor where metals are deposited on the large pore catalyst. Demetallized resid then flows to a second reactor where hydrogenation and molecular weight reduction occurs in the presence of the relatively smaller pore hydrogenation catalyst. Other somewhat similar two-stage upgrading processes are disclosed in U.S. Pat. Nos. 4,016,067; 4,212,729; 4,447,314 and 4,626,340. While multi-stage, multiple catalyst systems such as these provide one solution to many of the problems encountered in single stage, single catalyst systems, refiners desire other, simpler methods for upgrading resid to more valuable, lighter molecular weight products.
Another solution to upgrading resid is to employ a multi-component catalyst system in which catalyst particles of two different types are used in the same reactor. Examples of this type of process are disclosed in our U.S. Pat. Nos. 5,009,771 and 5,100,855. In these systems, a first catalyst component typically employs a porous refractory inorganic oxide support having less than 0.1 cubic centimeters per gram of pores with a diameter less than 200 Angstroms (the "micropores"), a pore volume of less than 0.02 cubic centimeters per gram of pores having a diameter greater than about 800 Angstroms (the "macropores"), and a maximum average mesopore diameter (i.e. pore diameter between 200 and 800 Angstroms) of about 130 Angstroms. The second catalyst component typically exhibits a pore volume of greater than 0.07 cubic centimeters per gram of pores having a diameter greater than about 800 Angstroms. Either catalyst can employ a Group V, VIB or VIII metal in an amount ranging from about 0.4 to 8.0 weight percent of the metal calculated as an oxide. This system is believed to work by causing metals to be deposited within the relatively large pore catalyst component, thereby substantially decreasing metal deposition on the relatively small pore component. This is believed to permit the small pore component to effectively upgrade feedstock much longer than would be possible if the large pore metal scavenging catalyst was not employed to prevent the blocking of small pore catalyst pore entrances.
A preferred method for improving the performance of resid upgrading systems is to employ a single hydrogenation catalyst having a wide variety of pore diameters within a single catalyst support. One example of a resid upgrading catalyst of this type is disclosed in U.S. Pat. No. 4,434,048 to Schindler. Schindler teaches the use of a support having between about 0.25 and 0.40 cubic centimeters per gram of pores having a radius of less than 125 Angstroms, between 0.10 and 0.25 cubic centimeters per gram of pores having a radius between 125 and 250 Angstroms, between 0.20 and 0.30 cubic centimeters per gram of pores having a radius between 250 and 750 Angstroms, between 0.05 and 0.15 cubic centimeters per gram of pores having a radius between 750 and 2000 Angstroms, and between 0.03 and 0.10 cubic centimeters per gram of pores having a radius greater than 2000 Angstroms. On this support is deposited between 1 and 6 weight percent nickel and between 5 and 16 weight percent molybdenum. In some cases, the catalyst may also include between 1 and 6 weight percent cobalt. While Schindler's catalyst may be somewhat easier to use than the multiple catalyst systems already described, the volume of micropores having a radius less than 125 Angstroms is believed to be substantially less than required for successful upgrading of resid in modern ebullated bed resid hydrotreating reactors of the types described herein.
Another example of a catalyst suitable for upgrading resid is that disclosed in our U.S. Pat. No. 4,707,466 to Beaton et al. This catalyst employs a porous inorganic support typically having at least 0.15 cubic centimeters per gram of pores with a diameter greater than 1200 Angstroms and about 0.5 cubic centimeters per gram of pores having a diameter of less than 150 Angstroms. Deposited on the support is about 3.5 to 5 weight percent of a Group VIB metal oxide and 0.4 to 0.8 weight percent cobalt oxide. We have found this catalyst to be particularly effective in ebullated bed reactor systems such as those disclosed in our U.S. Pat. Nos. 4,940,529 and 5,013,427 which are hereby incorporated by reference. As referred to herein, ebullated bed systems are those in which solid catalyst particles are kept in motion by the upward movement of liquids and gases. In these systems, a resid feedstock typically is converted to lighter products by ebullating a mixture of resid, hydrogen, recirculated liquid product and supported catalyst at temperatures around 800.degree. F. and at total system pressures around 3000 psia in one or more reactors.
Another way to improve resid hydrotreating performance is to employ a catalyst having an incremental pore distribution specifically suited to resid hydroprocessing. One such catalyst is disclosed in our allowed U.S. patent application Ser. No. 07/857,336, now U.S. Pat. No. 5,221,651. This particular catalyst is characterized by a support having a maximum value of incremental pore volume at a pore radius of about 30 Angstroms. On this support is deposited between about 0.4 and 8 weight percent of a Group VIII metal and 3 to 22 weight percent of a Group VIB metal, both metals being measured as an oxide. The incremental pore volume noted above is calculated by dividing the change in pore volume for a given measurement increment by the change in pore radius for that increment to yield dV/dr, the rate of change of pore volume per gram with pore radius, and then multiplying dV/dr by the average pore radius of the measurement increment in Angstroms to yield an incremental pore volume in cc/g. While this catalyst is believed to be well-suited to resid hydrotreating applications, interest remains high in other improved resid hydrotreating catalysts having different incremental pore volume distributions.
While the aforedescribed catalysts are believed to be particularly useful in ebullated bed resid hydrotreating reactors, we continue to search for improved catalysts that can provide for better operability and/or higher conversion under resid hydrotreating operating conditions.